1. Field of the Invention
This invention relates to optical couplers and more specifically to an optical coupler provided with an AC speed up circuit.
2. Description of the Prior Art
In the past, to couple a digital signal from one part of a system to another while maintaining electrical isolation between the two, capacitors and pulse transformers have been employed. They share the common disadvantage of having a high-pass filter characteristic and thus are unable to pass DC signals.
In more recent designs, optical coupling is often used. Optical coupling employs integrated opto couplers having input light emitting diodes optically coupled to output photo transistors. Such opto couplers pass DC signals with a high degree of electrical isolation but they have a severe limitation in their operational speed.
Many approaches have been attempted to decrease the response time of such opto couplers. One approach involves using an integrated output amplifier which reduces the propagation time to the order of 45 nanoseconds. Another approach is to use a circuit design which in some manner reduces the photo transistor's effective output capacitance (or the effect of the output capacitance) thus increasing the speed of the coupling. However, the response time is in the order of 100 nanoseconds.
The present invention provides the advantages of the DC opto coupler characteristic with the speed of AC coupling thus providing a response time that is substantially lower than that of the above circuits.